Thermosetting polymers and composites have been widely used in modern industry because of their low density, good mechanical properties, low cost, dimensional stability, and so on. Important monomers and oligomers for thermosets include unsaturated polyester, epoxy resin, vinyl ester, phenol-formaldehyde resin, melamine resin, etc. Traditionally, most of these resins have been synthesized using petroleum-based chemicals as the raw materials. However, due to the foreseeable limit of fossil feedstocks and the increasing environmental concerns, the polymer and composites industry is suffering from potential increasing cost and environmental regulations. Therefore, much effort has been devoted lately to develop polymers and composites from bio-renewable raw materials. See Wool et al. Bio-based Polymers and Composites, Elsevier, Amsterdam (2005); Belgacem et al., Monomers, Polymers, and Composites from Renewable Resources, Elsevier, Amsterdam (2008); Raquez et al. Prog. Polym. Sci. 35:487-509 (2010). The fluctuating price of petroleum-based products and stricter environmental rules and regulations increases the demand for bio-based products. Compared to petroleum-based products, bio-based products are environmentally friendly, biodegradable, sustainable, and versatile, in use. See Deka et al., Progress in Organic Coatings 66:192-192 (2009). In 2004, the world production from major oils totaled 380 million metric tons. Production has continued to rise at a rate of 3-4% per year with soybean oil the major oil produced. See Behera et al., Journal of Applied Polymer Science 109:2583-2590 (2008).
Plant oils are one of the most important bio-renewable chemical feedstocks for the polymer industry because of their high annual production, high availability, low toxicity, relatively low cost, and biodegradability. Plant oils and their derivatives have been widely used for the production of paints and coatings since the development of drying oil resins, taking advantage of the autoxidation crosslinking of the double bonds in the fatty acid chains. See Meier et al., Chem. Soc. Rev. 36:1788-1802 (2007); Xia et al., Green Chem. 13:1983-1909 (2010). During the last decade, a variety of plant oil-based polymers have been developed via free radical or cationic homo-polymerization, as well as copolymerization with petroleum-based monomers, such as styrene and divinylbenzene. See Lu et al., ChemSusChem 2:136-147 (2009); Li et at, Biomacromolecules 4:1018-1025 (2.003); Kunclu et at, Biomacromolecules 6:797-806 (2005); Henna et al., J. Appl. Polym. Sci. 104:979-985 (2007); Valverde et al., J. Appl. Polym. Sci. 107:423-430 (2008); Andjelkovic et. al., Polymer 46:9674-9685 (2005); Andjelkovic et al., Biomacromolecules 7:927-936 (2006); Lu et al., Biomocromolecules 7:2692-2700 (2006). Bio-based vegetable oils (e.g., soybean oil) contain triglycerides that are composed of three unsaturated fatty acid chains joined at a glycerol junction. See Fu et al., Journal of Applied Polymer Science 117:2220-2225 (2010). However, due to the relatively low reactivity of the double bonds in the fatty acid chain, some chemical modifications are usually needed to introduce reactive functional groups having higher reactivity. A widely explored method for the modification of plant oils involves the conversion of the double bonds to epoxy groups by using peracids and hydrogen peroxides. For example, the structure of epoxidized sucrose soyate is shown in FIG. 1. With the epoxidation polymerization occurs quickly with highly cross linked networks. See Behera et al., Journal of Applied Polymer Science 109:2583-2590 (2008); Kolot et al., Journal of Applied Polymer Science 91:3835-3843 (2003). Epoxidized plant oils have been utilized for coatings and composites by using conventional epoxy curing agents, such as amine and anhydride. However, the internal epoxy groups in epoxidized plant oils are much less reactive than the terminal epoxy groups in benchmark materials, for example bisphenol-A epoxy. Therefore, epoxidized plant oils have been further modified via the ring-open reaction of epoxy with unsaturated acids or alcohols to produce (meth)acrylated plant oils or plant oil-based polyols. These derivatives have been widely used to generate thermosets by free radical cure or hydroxyl-isocyanate reactions. See Wu et al., Polym. Int. 60:571-575 (2011); Lu al., Polymer 46:71-80 (2005); La Scala et al., Polymer 46:61-69 (2005); Can et al., J. Appl. Polym. Sci, 81:69-77 (2001); Pfister et al., ChemSusChem 4:703-717 (2011); Desroches et al., Polym. Rev. 52:38-79 (2012); Lu et al., Biomacromolecules 8:3108-3114 (2007); Lu et at, ChemSusChem 3:329-333 (2010); Polym. Rev. 48:109-155 (2008). Besides epoxidization, plant oils have also been modified by hydroformylation (see Petrovic et al., Polym. Int. 57:275-281 (2008); Petrovic et al., Eur. J. Lipid Sci. Technol. 112:97-102 (2010)) and thiol-ene reactions. See Meier et al., Macromol. Rapid commun. 31:1822-1826 (2010); Turunc et al., Green Chem. 13:314-320 (2011); Wu et al., ChemSusChem 4:1135-1142 (2011).
The biggest obstacle in the application of plant oils for the generation of polymers is the flexibility of the fatty acid chain, which leads to low glass transition temperature (Tg) and low mechanical properties such as modulus and hardness. Thus, plant oils cannot be used by themselves as structural and engineering materials. To overcome these limitations, petroleum-based monomers (for example styrene) are usually introduced to reinforce plant oil-based polymers, but the bio-renewable content is sacrificed to achieve the desired material properties in this approach. See Li et al., Biomocromolecules 4:1018-1025 (2003); Khot et al., J. Appl. Polym. Sci. 82:703-72.3 (2011).
Sucrose is a bio-renewable polyol that is naturally present in a variety of plants. Sucrose ester is a vegetable oil composed of sucrose and fatty acids that is frequently used as a bio-based curable material for decades. See Jinli et al., Chinese Journal of Chemical Engineering 17:1033-1037 (2009). The fatty esters of sucrose were first explored as a coating resin in 1960's. See Bobalek et al., Official Digest 33:453-468 (1961); Walsh et al., Div. Org. Coatings Plastic Chem. 21:125-148 (1961). However, a high degree of substitution of sucrose with fatty acids had not been achieved until an efficient process was developed by Procter & Gamble (P&G) Chemicals in 2002. See U.S. Pat. Nos. 6,995,232; 6,620,952; 6,887,947. In spite of a relatively high degree of substitution (average of 7.7 fatty acid chains per molecule) and moderate molecular weight (around 2,400 g/mol), sucrose esters of fatty acids have low viscosity (300-400 mPa·s) due to their compact architectures. Highly substituted sucrose esters of fatty acids (SEFAs) have been successfully commercialized under the brand Sefose®, and utilized as a diluent in alkyd resins by CCP. Furthermore, SEF As are a highly tunable platform such that a variety of derivatives and formulations with different properties and application can be developed, since many of the modification approaches for plant oils are applicable to sucrose esters.
Vegetable oils have been modified using maleinization, epoxidation, acrylation, and hydroxomethylation. See Fu et al., Journal of Applied Polymer Science 117:2220-2225 (2010). Epoxidation and acrylation are the most common forms of modification of soybean oils with acrylated soybean oils (ASO) being the most prevalent. See Bunker et a Journal of Oil and Colour Chemists's Association 83:460 (2000). Soybean oils have a wide distribution of functional groups, 0-9 polymerizable groups per molecule. Thus, allowing for substances to react readily to them. The double bonds and highly reactive end groups allows for free radical polymerization to occur. See Fu et al., Journal of Applied Polymer Science 117:2220-2225 (2010); Behera et al., Journal of Applied Polymer Science 109:2583-2590 (2008). Acrylated soybean oils (ASO) are used to form solvent-free curing of films, adhesives, coatings, inks, and varnishes. Acrylation of ESO will lead to great improvement of photoactivity because of the short time it takes to form crosslinks under ultraviolet radiation. See Pelletier et al., Journal of Applied Polymer Science 99:3218-3221 (2005).
Recently, Pan et al. reported the synthesis of a series of thermosets based on SEFAs. Epoxidized sucrose esters of fatty acids (ESEFAs) were prepared via epoxidization, and cured using cyclic anhydrides. See Pan et al., Green Chem. 13:965-975 (2011); Pan et al., Biomacromolecules 12;2416-2428 (2011); Pan et al., Macro. Rapid Comm. 32:1324-1330 (2011).
ESEFAs may be further derivatized by the reaction of epoxy with acid or alcohol, which can generate bio-based polyols for the application of polyurethane coatings and composites. See Pan et al., ChemSusChem 5:419-429 (2012). Compared to their counterparts based on triglyceride oils, thermosets based on SEFAs showed much higher Tgs and mechanical properties, which can be attributed to the high functionality of SEFA-based resins and the rigidity of the sucrose core. See Pan et al., Prog. Ord. Coat. 73:344-354 (2012); Pan et al., Green Chem. 13:965-975 (2011); Pan et al., Biomacromolecules 12:2416-2428 (2011); Pan et al. Macro. Rapid Comm. 32:13244330 (2011); Pan et al. ChemSusChem 5:419-429 (2012); Van et al., Polym. Int. 61:602-608 (2012); Nelson et al., J. Renew. Mater., 1:141-153 (2013); Nelson et al., J. Coat. Technol. Res. 10:757-767 (2013); Nelson et al., J. Coat. Technol. Res. 10:589-600 (2013).
With increased environmental regulations and the depleting petroleum reserves, a bio-based resin that has comparative properties to conventional dental restoration materials is also necessary. See Deka et al., Progress in Organic Coatings 66:192492 (2009). Improvement in ultraviolet light initiation has advanced the formulation of dental resin composites. See Furuse et al., Dental Materials 27:497-506 (2011). The three leading restorative materials are ethyl methacrylates, methyl methacrylates, and bis-acrylate resin composites. For the past 40 years bisphenol-A diglycidylether methacrylate (bis-GMA) has been the main composite system. What makes bis-GMA stand out among other composites is its high viscosity and rigid central structure, which reduces its ability to rotate and participate in the polymerization process. See Yap et al., Dental Materials 20:370-376 (2004); Prakki et al., Dental Materials 23:1199-1204 (2007). Because of such high viscosity, bis-GMA is normally diluted with triethyleneglycol dimethacrylate (JEGDMA). FIG. 2 shows the structures of bis-GMA and TEGDMA. TEGDMA works not only to reduce viscosity of bis-GMA, but also to increase the methacrylate double bond conversions. See Mahmoodian et al., Dental Materials 24:514-521 (2008).
Because the viscosity of bis-GMA is so high, handling it is hard and unfavorable. See Mahrnoodian et al., Dental Materials 24:514-521 (2008). In addition, the safety of bisphenol-A (BPA) has come under speculation, causing people to be unsure about using a product that contains BPA, including dental composites. See Kovacic, Medical Hypotheses 75:1-4 (2010).
The invention provides the synthesis of methacrylated epoxidized sucrose soyate (MAESS) oligomers. MAESS oligomers may be formulated into thermosets and cured via a free radical mechanism using styrene, for example, as the reactive diluent. The bio-based thermosets have beneficial thermal and mechanical properties. In addition, the invention relates to a vegetable oil resin that compares favorably to the properties of BPA, and is an appropriate bio-based substitute for the currently used petroleum-based dental resins.